Optical coherence tomography (OCT) as a noninvasive imaging method is mainly used in medicine. Here, the different reflection and scattering properties of different tissues form the basis of this method. As a relatively new method, OCT is currently establishing itself in more and more fields of application.
What is optical coherence tomography?
In the field of ophthalmic diagnostics, OCT proves to be very beneficial, here mainly the fundus of the eye mot OCT is examined. The physical basis of optical coherence tomography is the formation of an interference pattern during the wave superposition of reference waves with reflected waves. The decisive factor is the coherence length of the light. The coherence length represents the maximum travel time difference of two light beams that still allows the formation of a stable interference pattern when they are superimposed. In optical coherence tomography, light with a short coherence length is used with the aid of an interferometer to determine the distances of scattering materials. For this purpose, the area of the body to be examined is scanned in a point-like manner in medicine. The method allows a good depth examination due to the high penetration depth (1-3 mm) of the radiation used into the scattering tissue. At the same time, there is also a high axial resolution at a high measurement speed. Optical coherence tomography thus represents the optical counterpart of sonography.
Function, effect, and goals
The optical coherence tomography method is based on white light interferometry. It uses the superposition of reference light with reflected light to form an interference pattern. In this way, the depth profile of a sample can be determined. For medicine, this means the examination of deeper tissue sections that cannot be reached with classical microscopy. Two wavelength ranges are of particular interest for the measurements. One is the spectral range at a wavelength of 800 nm. This spectral range provides good resolution. On the other hand, light with a wavelength of 1300 nm penetrates particularly deep into the tissue and allows a particularly good depth analysis. Today, two main OCT application methods are used: Time Domain OCT systems and Fourier Domain OCT systems. In both systems, the excitation light is split into reference and sample light via an interferometer, resulting in interference with the reflected radiation. Lateral deflection of the sample beam over the area of interest produces cross-sectional images, which are fused to produce an overall image. The Time Domain OCT system is based on short-coherent, broadband light, which produces an interference signal only when both arm lengths of the interferometer match. Thus, the position of the reference mirror must be traversed to determine the backscatter amplitude. Due to the mechanical movement of the mirror, the time required for imaging is too high, so this method is not suitable for fast imaging. The alternative method of Fourier Domain OCT works on the principle of spectral decomposition of the interfered light. This simultaneously captures the entire depth information and significantly improves the signal-to-noise ratio. Lasers are used as light sources, which scan the body parts to be examined step by step. The areas of application of optical coherence tomography are primarily in medicine and here especially in ophthalmology, cancer diagnostics and skin examination. The different refractive indices at the interfaces of the tissue sections concerned are determined via the interference patterns of the reflected light with the reference light and displayed as an image. In ophthalmology, mainly the fundus of the eye is examined. Competing techniques, such as the confocal microscope, cannot adequately image the layered structure of the retina. Other techniques sometimes place too much strain on the human eye. Especially in the field of ophthalmic diagnostics, OCT therefore proves to be very advantageous, especially since the non-contact measurement also eliminates the risk of infection and psychological stress. Currently, new perspectives are opening up for OCT in the field of cardiovascular imaging. Intravascular optical coherence tomography is based on the use of infrared light. Here, OCT provides information about plaques, dissections, thrombi or even stent dimensions.It is also used to characterize morphological changes in blood vessels. In addition to medical applications, optical coherence tomography is increasingly conquering fields of application in materials testing, for monitoring production processes or in quality control.
Risks, side effects and hazards
Compared to other methods, optical coherence tomography has many advantages. It is a noninvasive and noncontact method. This allows it to largely avoid the transmission of infections and the occurrence of psychological stress. Furthermore, OCT does not use ionizing radiation. The electromagnetic radiation used largely corresponds to the frequency ranges to which humans are exposed on a daily basis. A major advantage of OCT is also that the depth resolution is not dependent on the transverse resolution. This eliminates the need for thin sections used in classical microscopy because the technique is based on pure optical reflection. Thus, microscopic images can be generated in living tissue due to the large penetration depth of the radiation used. The operating principle of the method is very selective, so that even very small signals can be detected and assigned to a specific depth. For this reason, OCT is also particularly well suited for examining light-sensitive tissue. The use of OCT is limited by the wavelength-dependent penetration depth of the electromagnetic radiation and the bandwidth-dependent resolution. However, broadband lasers have been developed since 1996, which have further advanced the depth resolution. Thus, since the development of UHR-OCT (ultra-high resolution OCT), even subcellular structures in human cancer cells can be imaged. Since OCT is still a very young technique, not all possibilities have been exhausted yet. However, optical coherence tomography is attractive because it poses no health risk, has very high resolution, and is very fast.
